(a) Field of the Invention
The present invention relates to a control method for an automatic transmission, and more particularly, to a control method for an automatic transmission in which an operational range of a damper clutch is increased when in a fourth speed, power off state.
(b) Description of the Related Art
General control of an automatic transmission includes control to minimize torque fluctuations and improve shift quality. In particular, such control reduces engine output torque when it is necessary to retard ignition timing or cut off fuel such that shift shock is reduced and a clutch wears more slowly.
With reference to FIG. 4, in a power off state in which engine torque is less than output shaft load (i.e., where a throttle valve is closed), the conventional control method performs control such that a damper clutch is set on non-operational slip control. That is, the damper clutch is controlled to non-operational slip control at a first point P1 (i.e., power off), over an entire drive range. Next, at a second point P2, which is an end of an interval t1 where a torque valve opening becomes 0% and fuel supply is reduced, a fuel cut-off interval t2 is started in which fuel supplied to the engine is cut off. At roughly the second point P2 of the fuel cut-off interval t2, engine rpm Ne become less than turbine rpm Nt of a torque converter.
At a third point P3, which is at an end of the cut-off interval t2 or where engine rpm Ne are approximately 1200, engine rpm Ne are slightly increased, again decreased until reaching an idle state, then maintained at this level. This slight increase in engine rpm Ne, which is effected by an increase in the supply of fuel at the designated point, is done to prevent engine stall.
However, with the use of the above fuel cut-in control to prevent engine stall, because of the difference between engine rpm and turbine rpm, backlash shock is generated in a powertrain and noise is generated as a result of the backlash shock when converting to power off from power on.
The present invention has been made in an effort to solve the above problems.
It is an object of the present invention to provide a control method for an automatic transmission in which an operational range of a damper clutch is increased when in a fourth speed, power off state, thereby improving shift feel and reducing fuel consumption.
To achieve the above object, the present invention provides a control method for an automatic transmission comprising the steps of (a) determining if the automatic transmission is in a fourth speed and the vehicle is being driven in a power off state; (b) outputting a damper clutch operation slip control signal if the vehicle is in a power off state; (c) outputting an engine fuel cut-in rpm reduction signal to engine control means if a damper clutch is operated by the damper clutch operation slip control signal; (d) receiving a lower engine fuel cut-in signal from the engine control means according to the output engine fuel cut-in rpm reduction signal; (e) determining if engine output is undergoing continuous reduction; and (f) controlling a duty ratio using a learned value during 4-3 shifting if engine output is being reduced.
According to a feature of the present invention, the transmission is controlled by a conventional method if the vehicle is not in a power off state in step (a).
According to another feature of the present invention, if it is determined that engine output is not being reduced in step (e), an engine fuel cut-in rpm reduction discontinue signal is output to the engine control means, after which the automatic transmission is controlled using the conventional method.
According to yet another feature of the present invention, the engine fuel cut-in rpm reduction discontinue signal is a signal for converting the reduced engine fuel cut-in rpm to the engine fuel cut-in rpm conventionally used.
According to still yet another feature of the present invention, the engine fuel cut-in rpm reduction signal is a signal for reducing engine output to a predetermined engine fuel cut-in rpm.
According to still yet another feature of the present invention, the conventional engine fuel cut-in rpm are 1,200 rpm and the predetermined engine fuel cut-in rpm are 1,000 rpm